1. Related Application
The present application is related to the subject matter of U.S. patent application Ser. No. 09/350,744, filed Jul. 9, 1999.
2. Background of the Invention
Many manufacturing processes require accurate and repeatable pressure measurements during critical process steps. These processes may rely on capacitance diaphragm gauges to achieve an accurate determination of process chamber pressure. Capacitance diaphragm gauges (or capacitance manometers) are widely used in the semiconductor industry. In part, this is because they are typically well suited to the corrosive services of this industry. They are also favored because of their high accuracy and immunity to contamination.
A capacitance manometer is a type of sensor which may be used to measure parameters such as the pressure within a process chamber. A capacitance manometer has a housing containing two chambers separated by a diaphragm. One of the chambers is in fluid communication with the process chamber or conduit in which the pressure is to be measured.
The other chamber of the manometer is a typically (although not necessarily) evacuated. It is a pressure reference chamber. Plates are located on the manometer housing and on the diaphragm. These plates have a capacitance that can be measured. When the process gas enters the first chamber, it exerts a pressure against the diaphragm and causes the diaphragm to move. The capacitive plate connected to the diaphragm is consequently moved toward the plate connected to the manometer housing, changing the capacitance between the plates. The change in capacitance corresponds to the increase in pressure and can be used as a measurement of the pressure.
Capacitance manometers typically operate by measuring the change in electrical capacitance that results from the relative movement of the sensing electrodes. The change in capacitance can be measured using various different types of electrical interfaces, such as balanced diode bridge interfaces, guarded secondary transformer-based bridge interfaces, and matched reference capacitor bridge interfaces. These interfaces measure changes in capacitance, using circuitry coupled to the capacitive plates of the manometer in order to determine changes in their capacitance and corresponding changes in the measured parameter.
One of the major advantages of a capacitance diaphragm gauge is its ability to detect extremely small diaphragm movements, hence extremely small changes in the measured process parameter. The accuracy of these sensors is typically 0.25 to 0.5% of the generated reading. For example, in a typical capacitance diaphragm pressure sensor, a thin diaphragm can measure down to 10−5 Torr. Thicker, but more rugged diaphragms can measure in the low vacuum to atmospheric range. To cover a wide vacuum range, two or more capacitance sensing heads can be connected into a multi-range package.
Systems that utilize differential capacitance manometers generally have stringent requirements for the repeatability of pressure readings, with offset drift typically limited to 0.02% of full scale per day. Full scale deflection for a differential capacitance manometer typically causes capacitance changes of 0.2 2.0 pF (10−12 F.). Thus, the electronic interface (“Analog Front End” or “AFE”) to the sensing element may not experience drift in excess of 0.04 femtoFarad (10−15 F.) per day.
In addition to stringent performance requirements, customers are increasingly requiring features that allow differential capacitance manometer based systems to take advantage of advancements in other process equipment. For example, digital communications, embedded diagnostics and lower temperature sensitivity are now required by some of the latest process technologies. Legacy capacitance diaphragm gauges often cannot meet these requirements.